This invention relates generally to the low cycle fatigue life of the bore area of a rotating disk and, more particularly, to a novel method of enhancing (i.e., extending) said life.
In some applications (such as are in use in advanced, high tip-speed, gas turbine engines), the bore area of a rotating disk is subjected to exceptionally high stresses. The result is that such a bore area can become low cycle fatigue life limiting to the same extent as stress concentration areas, such as bolt holes, dovetails, rabbets, and the like. In a gas turbine engine, the consequence of failure of a bore area of the rotating disk is far more severe than a failure in the web/rim area of the rotating disk. For example, in an aircraft engine of the gas turbine type, the failure of the bore area in a high pressure turbine disk would result in the release of 100 times the amount of kinetic energy that would be released if a single post (with 2 blades) in the rim area of the same disk failed. It is imperative, therefore, to have additional margin in the low cycle fatigue life of the bore area of a rotating disk.
The concept of prestressing the bore area of a rotating disk to improve low cycle fatigue life is not new. Some of the prior art methods include: conventional overspeed; mandrel expansion of the bore by differential heating/cooling (i.e., uniform heating of the disk and cooling of the mandrel); the use of an interference fit bore sleeve; and the use of an interference fit bore sleeve plus bore prestress. All of the prior art methods have some inherent disadvantages, such as being complex, costly, and compromising the disk and/or the turbomachine (e.g., the engine) design. For example, conventional overspeeding of the disk will induce some desired residual compressive stress in the bore area, but it will also give an undesirable level of residual tensile stress in the rim area.
It is, therefore, readily apparent that what is needed in the art, and is not currently available, is a method of enhancing the low cycle fatigue life of the bore area of a rotating disk without any of the inherent disadvantages of the prior art methods.